The mortality and morbidity associated with neonatal bacterial meningitis remain significant despite advances in antimicrobial chemotherapy and supportive care. A major contributing factor is the incomplete understanding of the pathogenesis of this disease. E. coli is the most common gram-negative organism that causes meningitis during the neonatal period. Most cases of E. coli meningitis in newborns develop as a result of hematogenous spread, but it is not completely understood how circulating E. coli cross the blood-brain barrier. We have developed an in vitro model of the blood-brain barrier with human brain microvascular endothelial cells (HBMEC) and an infant rat model of experimental hematogenous E. coli meningitis, which mimics human E. coli meningitis (e.g., hematogenous infection of the meninges). Using these in vitro and in vivo systems, we have shown that E. coli K1 traverses the blood-brain barrier without altering the integrity of HBMEC monolayers and inducing a change in blood-brain barrier permeability. During the previous funding period, we showed that E. coli K1 invades HBMEC through a zipper-like mechanism and transmigrates through an enclosed vacuole without intracellular multiplication. We identified that several E. coli K1 determinants contribute to HBMEC invasion in vitro and crossing of the blood-brain barrier in vivo and some of the E. coli proteins interact with the specific receptors present on HBMEC. We also showed that the K1 capsule has a novel role in E. coli traversal of the blood-brain barrier as live bacteria. Based on these findings, we would like to examine the following specific aims. 1.To continue to characterize microbial determinants contributing to E. coli K1 invasion of HBMEC in vitro and traversal of the blood-brain barrier in vivo. 2.To examine the mechanisms involved in E. coli K1 invasion of HBMEC by identifying and characterizing HBMEC receptors as well as by determining host cell signal transduction pathways. 3.To determine the role of K1 capsule in E. coli trafficking in HBMEC. Further understanding and characterization of E. coli K1-HBMEC interactions should allow us to develop novel strategies to prevent this serious infection.